PM6670S View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'PM6670S' PDF : 53 Pages View PDF
Application information
PM6670S
where
Equation 27a
α OSC
=
VOSC
VIN
Equation 27b
α OUT
=
VSNS
VOUT
Referring to the typical application schematic (figure in cover page and Figure 29), the final
expression is then:
Equation 28
fSW
= αOSC
K OSC
= R2
R1 + R2
⋅1
K OSC
The switching frequency directly affects two parameters:
● Inductor size: greater frequencies mean smaller inductances. In notebook applications,
real estate solutions (i.e. low-profile power inductors) are mandatory also with high
saturation and r.m.s. currents.
● Efficiency: switching losses are proportional to the frequency. Generally, higher
frequencies imply lower efficiency.
Even if the switching frequency is theoretically independent from battery and output
voltages, parasitic parameters involved in power path (like MOSFETs on-resistance and
inductor DCR) introduce voltage drops responsible for a slight dependence on load current.
In addition, the internal delay is due to a light dependence on input voltage.
Table 12. Typical values for switching frequency selection
R1 (kΩ)
R2 (kΩ)
Approx switching frequency (kHz)
330
11
250
330
13
300
330
15
350
330
18
400
330
20
450
330
22
500
40/54
Doc ID 14432 Rev 4
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